(1) Field of the invention
The present invention relates to a scanning type scintillation camera, and more particularly it pertains to a scanning type scintillation camera arranged so that an effective area of observation is expanded relative to the actual area scanned by the detector.
(2) Description of the prior art
A scanning type scintillation camera, in general, is designed so that bright spots of radiation are detected while moving the detector of the camera relative to a subject under examination such as a human body, and that the positions of occurrence of bright spots noted in the coordinate system set for the electrically formulated first spatial "window" (the word "window" herein used is a technical term known in this field of technique, and will hereinafter be mentioned without such adjective as "spatial" for the sake of simplicity) of the camera are converted to coordinate system intended for imaging the whole body of the subject by adding them to or subtracting them from the distance covered by the detector, to thereby obtain a scintigram through photography. This prior art will be explained more concretely hereinbelow by referring to FIG. 1. In this Figure, reference numeral 1 represents a detector of a scintillation camera for detecting radiation. Symbols X and Y represent an X-axis and a Y-axis of a coordinate system which, herein, is orthogonal coordinate system intended for imaging the whole body under examination. The detector 1 is understood to move in the direction of the X-axis at a constant speed. 2 represents a "window" as briefed above which may be, for example, a rectangular window. Symbol a represents the width in the X-axis direction of this rectangular window. Symbol b represents the length in the Y-axis direction thereof. 1' represents a position of the detector 1 assumed at the end of its movement. Let us now assume that the detector 1 is located at a position X.sub.c, Y.sub.c in the coordinate system for imaging the whole body. When the detector 1 detects the fact that radiation such as a gamma ray has impinged onto points x, y of the coordinate system set for the detection range of the scintillation camera, there are outputted coordinate signals (x, y) from the scintillation camera. These signals are added to the positional signals (X.sub.c, Y.sub.c) in the coordinate system of the detector intended for imaging the whole body, and the following processing by computation EQU X=X.sub.c +x
(1) EQU Y=Y.sub.c +y
is carried out, so that these signals (X, Y) will provide positional signals corresponding the bright spots of radiation in the coordinate system for imaging the whole body. Then, these positional signals (X, Y) to which said signals X.sub.c, Y.sub.c have been converted are inputted to a display X, Y oscilloscope to form bright spots at positions corresponding to those positions in the coordinate system intended for imaging the whole body. This cycle of operation is repeated for each detection of gamma ray by the detector 1 as the latter moves at a constant speed. By recording these bright spots on a film, it is possible to record the distribution of radiation density of the particular area scanned by the detector 1. The processing by computation shown in Formula (1) is carried out only for that gamma ray which has impinged within the area a.times.b of the window 2 of the detector 1 shown in FIG. 1. Those gamma rays which have impinged onto those areas located outside this area of the window 2 are excluded, to thereby keep the sensitivity of the scintillation camera constant.
Let us now assume that, in FIG. 1, the detector 1 starts scanning in the X-axis direction at a constant speed from a point on the X-axis and represented by X.sub.c =a/2, Y.sub.c =0 (which means point P in FIG. 2), and that it has performed scanning up to a point located on the X-axis and represented by X.sub.c =X.sub.E -(a/2) (which means point Q in FIG. 2). The time T required by the aforesaid window of the scintillation camera for the observation of the respective points located in the area scanned by the detector 1 will become as shown in FIG. 2. More particularly, the time T which is required by the window 2 having a width a in the X-axis direction to make the observation of those points located between point P' (which is represented by X.sub.c =a) and point Q' (which is represented by X.sub.c =X.sub.E -a) is T=a/V. However, the time T which is required by the window 2 to make the observation of points located on the X-axis at positions ahead of point P' or to make the observation of points located on the X-axis at positions behind Q' will have a value smaller than a/V as shown in FIG. 2. In this Figure, the distance D from point P to point Q is D=X.sub.E -a which represents the area scanned actually by the detector 1. And, the distance from point P' to point Q', i.e. D.sub.eff =X.sub.E -2a, will be the actually effective area for observation. It should be understood, however, that the above-mentioned relationship is one which is obtained in case the detector 1 scans at a constant speed the scanning distance D from point P to point Q. In actual operation, it is impossible to sharply raise the speed of movement of the detector 1 from its rest state up to its gaining a constant speed. Therefore, the difference between the distance covered by the detector 1, i.e. said area of scanning D, and the effective area of observation will become further greater.
Next, the sequential steps of forming a scintigram by the use of a conventional scanning type scintillation camera will be explained by referring to the schematic illustration in blocks shown in FIG. 3. In this Figure, 11 represents a detector of a scintillation camera. 12 represents a pedestal supporting the detector 11. 13 represents a bed of a patient or a subject under examination. 14 represents the patient. The gamma ray which has been detected by the detector 11 is transmitted to a console (operation table) 15 of the scintillation camera, whereat the detected gamma ray is subjected to computation with respect to the position of its incidence, and the computed position of the gamma ray is outputted as positional signals (x, y) in the coordinate system which uses the center of the detector as the point of origin. 16 represents a scanning speed control circuit for controlling the scanning speed of the scintillation camera. Here, in order to simplify the explanation, the direction of movement of the detector 11 is assumed to be the x-axis direction of the x, y corrdinate system of the detector. 17 represents a window circuit which outputs only those input signals, among the coordinate signals x, y which are outputted from the console 15, which enter in the region represented by .vertline.x.vertline.&lt;(a/2) and .vertline.y.vertline.&lt;(b/2). 18 represents a coordinate conversion circuit which receives, as its inputs, positional signals X.sub.c, Y.sub.c of the detector in the whole body imaging coordinate system outputted from a mechanism not shown for reading the position of the detector and also signals x, y outputted from the window circuit 17 and which outputs coordinate conversion signals X, Y which are obtained through the computation X=X.sub.c +x, Y=Y.sub.c +y. 19 represents a limiter circuit which is intended to preclude those data locating outside the effective obervation area D.sub.eff (the area from point P' to point Q' in FIG. 2), since they cannot be used as imaging data. The output from this limiter circuit 19 is inputted to an X, Y oscilloscope 20 to thereby form a bright spot at a site corresponding to the position of the incident gamma ray, and thus a scintigram is recorded on the film of the camera 21.
As will be understood from the foregoing description, let us now suppose that a scanning has been performed at a constant speed from point P represented by X.sub.c =(a/2) up to point Q represented by X.sub.c =X.sub.E -(a/2). Then, it will be understood that the respective positions falling within the effective observation area D.sub.eff represented by a.ltoreq.X.sub.c .ltoreq.X.sub.E -a are observed by the detector only for the length of time T=a/V which is determined by the width a of the window and the constant velocity V. With respect to those positions locating in the area represented by 0&lt;X.sub.c &lt;a (meaning the area ahead of point P') and those positions locating in the area represented by X.sub.E -a&lt;X.sub.c &lt;X.sub.E (meaning the area locating behind point Q'), their observation time T will become smaller than the aforesaid T=a/V because of the fact that the entire width a of the window does not pass them. Thus, those latter two groups of positions cannot be utilized as observation data. It is the role of the limiter circuit 19 to preclude the data concerning these two kinds of areas. As will be understood from the foregoing description, an effective observation area D.sub.eff is smaller by the width a of the window than the distance D actually covered by the detector.